1. Field of the Invention
The present invention relates to an auto-focusing system and, particularly, to an auto-focusing system for a video camera.
2. Description of the Prior Art
It is known, as a method of auto-focusing by utilization of the properties of the video camera, the step-by-step control method wherein the definition of the picture is detected using high frequency components of the video signal and the distance setting ring (will be termed "helicoid" hereinafter) of the lens is rotated under control so as to obtain the best definition. This method is described in detail in an article entitled, "Automatic Focus Adjustment for TV Camera by Step-by-step Servo System", by Ishida et al., NHK Technical Report, Vol. 17, No. 1 (1965), Serial No. 86, page 21. This method will first be explained briefly in connection with FIGS. 1 and 2.
FIG. 1 shows in block diagram the arrangement of the auto-focusing system using the step-by-step control method. The arrangement includes a lens system 1, a camera circuit 2, a video signal output terminal 3, a high-pass filter (HPF) 4, a detector 5, a difference holding circuit (serves as the combination of a differential circuit and a sample holding circuit) 6, a motor drive circuit 7, and a motor 8 for turning the helicoid in the lens system 1.
The operation of the arrangement shown in FIG. 1 will be described referring to the characteristic graphs in FIG. 2. The incident ray from an object is focused by the lens system 1, then converted into the electrical signal by the camera circuit 2, which provides at the terminal 3 the video signal for the object. The high-pass filter (HPF) 4 extracts high frequency components of the video signal and the detector 5 detects the high frequency components and provides a voltage signal at terminal 51. Since the voltage at the terminal 51 (will be termed "focus signal": curve (1) in FIG. 2) represents the definition of the picture, it becomes maximum when the helicoid position (A) of the lens system 1 is set exactly corresponding to the distance between the object and the lens system 1, while it decreases as the helicoid setting deviates from the object distance. In FIG. 2, curves (2) and (3) show the output signal level at terminal 61 when the helicoid position is moved from proximity to infinity and from infinity to proximity, respectively.
FIG. 2 suggests that if the helicoid position is controlled by some means as if one ascends the slope of the focus signal curve so as to lead the helicoid position to the peak where the focus signal has the maximum value, then automatic focusing will be achieved. This process is carried out by the circuit portion including the difference holding circuit 6, the motor drive circuit 7 and the motor 8 in FIG. 1. The motor 8 moves the helicoid position, while the difference holding circuit 6 samples and holds the focus signal on the terminal 51 at a certain interval and provides a positive voltage if the sampled signals show a positive variation (i.e. ascent) or provides a negative voltage if the sampled signals show a negative variation (i.e. descent). The motor drive circuit 7 keeps the turning direction of the motor 8 to ascend the slope of curve when the difference holding circuit 6 provides a positive voltage, or reverses the turning direction to take the ascending direction when the circuit 6 provides a negative voltage. Thus the helicoid position control loop shown in FIG. 1 leads the helicoid position to ascend the slope of the focus signal by referring to the output voltage of the difference holding circuit 6, and eventually the helicoid position will zigzag to the peak of the curve. By detecting the arrival at the peak point, the lens position is fixed and auto-focusing is completed.
The auto-focusing system by step-by-step control for a video camera has been described. This method uses the picture signal to carry out the focusing operation and realize an inexpensive and accurate auto-focusing system with the simpler structure and less initial adjustments as compared with the system having a closed-loop helicoid position control based on an independent distance metering device. However, the foregoing method has the following problems related to the dynamic range of the focus signal. The amplitude of the video signal from the camera circuit 2 is controlled to have a constant level by the automatic or manual gain control circuit within the camera circuit 2, while the output of the detector 5 is responsive to the amount of high frequency components included in the video signal, i.e., energy included in the output of the HPF 4. Therefore, if the picture has little high-contrast vertical line components, the detector 5 receives from the HPF 4 a less number of pulses with small amplitude, resulting in a small peak on the curve of the focus signal. On the other hand, if the picture has much high-contrast vertical line components, the detector 5 receives a large number of pulses with larger amplitude, resulting in a large peak on the curve of the focus signal. In the actual picture signal, there is a great difference between these peaks, that extends, according to an experiment, about 1 to 200.
Since the dynamic range of the focus signal is limited mainly by the power voltage, the detector 5 is designed so that it is not saturated for the abovementioned higher peak. Therefore, the detector 5 provides a lower voltage for a picture having less vertical line components, causing the difference holding circuit 6 to fail to operate, and the circuit does not provide the output voltage responsive to the full range of the focus signal. Conversely, if the detector 5 is designed so as to provide a sufficient output voltage for a picture having less vertical line components, it will be saturated by a high-contrast picture, resulting in a flattened peak on the curve of the focus signal. Also in this case, the difference holding circuit 6 does not receive a correct signal, and the accurate ascending operation cannot be expected.